1. Technical Field
The present disclosure relates to a detecting device for the midpoint voltage of a transistor half bridge circuit.
2. Description of the Related Art
The circuits with high voltage transistor half bridges are employed in various applications, such as motor control, ballast for fluorescent lamps and supply circuits. The half bridge circuits consist of a pair of stacked transistors placed between a high supply voltage Vin and the ground GND, as seen in FIG. 1.
The transistors Q1 and Q2 are power devices of MOSFET type and their common connection point, node “A”, is the output connected to the load. Each transistor Q1 and Q2 has its driving driver DRV1 and DRV2 providing the convenient signals for switching them on or off.
In many applications, monitoring the midpoint node, node A, is convenient for determining when this switched from a high state to a low state or vice versa, such as, for example, the LLC series resonant circuit 1 in FIG. 1. During the normal operation of the circuit, the transistors Q1 and Q2 are alternately switched on and off to make the current flow into the resonant load connected to node A. When transistor Q1 is switched on, the voltage of node A is brought to the high potential, substantially the supply voltage, and so the current starts increasing in the resonant load. When transistor Q1 is switched off, the current flowing into the resonant load causes the voltage of node A to decrease towards a low potential. The transistors Q1 and Q2 are assumed to switch at a frequency which is higher than the resonance frequency of the load circuit. After a certain idle time such to ensure that the voltage of node A has decreased to a low voltage which is typically 0 volts, i.e., the ground GND, the transistor Q2 is switched on. The idle time avoids both transistors Q1, Q2 from being simultaneously switched on, which would cause a short circuit between the high and low voltages; the idle time ensures that the transition of the voltage of node A, i.e., the voltage of the midpoint node, is already over before switching on the other transistor.
The complete transition from the high to the low voltages of the voltage of node A, before switching on the transistor Q2, will employ a finite time quantity. Under certain conditions, the voltage to node A could not be completely decreased to the low potential when the transistor Q2 is switched on. In this case, transistor Q2 will force the voltage of node A to the low voltage level. This switching is called “hard-switching” and it is a switching loss that will cause the transistors Q1 and Q2 to superheat such to even irreversibly damage the two transistors.
Hence, the switching losses may be minimized thus ensuring that node A has finished the transition from the high voltage to the low voltage or vice versa before switching on the transistors.